Heat your butter in a small saucepan over low heat until melted.
In the bowl of a stand mixer fitted with the whisk attachment, add the eggs and caster sugar and whisk on high until increased in volume, pale and fluffy, approx five minutes. Reduce the speed to low, and add the milk and vanilla extract and whisk till combined.
Sift the plain flour, cocoa powder, baking powder and salt into a separate bowl and then add to the mixture. Mix till just combined.
Remove the bowl from the stand mixer and pour in the melted butter and fold till combined. Chill the mixture in the refrigerator for one hour.
Prior to baking, pre-heat the oven to (180°C/350°F). Brush melted butter onto your moulds and then dust with cocoa powder and tap away excess. Refrigerate your pan until the mixture is ready.
Spoon a dollop of batter into the shell mould and then place 2 white chocolate buttons onto the mixture and cover with another dollop of the batter. Repeat with each shell mould.
Bake for 10-12 minutes until the sponge has risen and ‘domed’ as it is fully cooked through. Transfer to a wire rack and leave to cool slightly.
Heat the milk chocolate and coconut oil in short bursts in the microwave until fully melted. Dip the madeleines into the milk chocolate and sprinkle with flaked salt. Best eaten within a few hours of making.